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Summary: Ctors | Methods | Inherited Methods

# MutableCallSite

`public class MutableCallSite`
`` `extends CallSite` ``

java.lang.Object
↳ | java.lang.invoke.CallSite
| ↳ | java.lang.invoke.MutableCallSite

A `MutableCallSite` is a `CallSite` whose target variable behaves like an ordinary field. An `invokedynamic` instruction linked to a `MutableCallSite` delegates all calls to the site's current target. The CallSite#dynamicInvoker of a mutable call site also delegates each call to the site's current target.

Here is an example of a mutable call site which introduces a state variable into a method handle chain.

>     MutableCallSite name = new MutableCallSite(MethodType.methodType(String.class));
>      MethodHandle MH_name = name.dynamicInvoker();
>      MethodType MT_str1 = MethodType.methodType(String.class);
>      MethodHandle MH_upcase = MethodHandles.lookup()
>        .findVirtual(String.class, "toUpperCase", MT_str1);
>      MethodHandle worker1 = MethodHandles.filterReturnValue(MH_name, MH_upcase);
>      name.setTarget(MethodHandles.constant(String.class, "Rocky"));
>      assertEquals("ROCKY", (String) worker1.invokeExact());
>      name.setTarget(MethodHandles.constant(String.class, "Fred"));
>      assertEquals("FRED", (String) worker1.invokeExact());
>      // (mutation can be continued indefinitely)

The same call site may be used in several places at once.

>     MethodType MT_str2 = MethodType.methodType(String.class, String.class);
>      MethodHandle MH_cat = lookup().findVirtual(String.class,
>      "concat", methodType(String.class, String.class));
>      MethodHandle MH_dear = MethodHandles.insertArguments(MH_cat, 1, ", dear?");
>      MethodHandle worker2 = MethodHandles.filterReturnValue(MH_name, MH_dear);
>      assertEquals("Fred, dear?", (String) worker2.invokeExact());
>      name.setTarget(MethodHandles.constant(String.class, "Wilma"));
>      assertEquals("WILMA", (String) worker1.invokeExact());
>      assertEquals("Wilma, dear?", (String) worker2.invokeExact());

Non-synchronization of target values: A write to a mutable call site's target does not force other threads to become aware of the updated value. Threads which do not perform suitable synchronization actions relative to the updated call site may cache the old target value and delay their use of the new target value indefinitely. (This is a normal consequence of the Java Memory Model as applied to object fields.)

For target values which will be frequently updated, consider using a VolatileCallSite instead.

## Summary

### Public constructors
`MutableCallSite(MethodType type)` Creates a blank call site object with the given method type.
`MutableCallSite(MethodHandle target)` Creates a call site object with an initial target method handle.

### Public methods
`final MethodHandle` | `dynamicInvoker()` Produces a method handle equivalent to an invokedynamic instruction which has been linked to this call site.
`final MethodHandle` | `getTarget()` Returns the target method of the call site, which behaves like a normal field of the `MutableCallSite`.
`void` | `setTarget(MethodHandle newTarget)` Updates the target method of this call site, as a normal variable.

### Inherited methods
From class `java.lang.invoke.CallSite`
`abstract MethodHandle` | `dynamicInvoker()` Produces a method handle equivalent to an invokedynamic instruction which has been linked to this call site.
`abstract MethodHandle` | `getTarget()` Returns the target method of the call site, according to the behavior defined by this call site's specific class.
`abstract void` | `setTarget(MethodHandle newTarget)` Updates the target method of this call site, according to the behavior defined by this call site's specific class.
`MethodType` | `type()` Returns the type of this call site's target.
From class `java.lang.Object`
`Object` | `clone()` Creates and returns a copy of this object.
`boolean` | `equals(Object obj)` Indicates whether some other object is "equal to" this one.
`void` | `finalize()` Called by the garbage collector on an object when garbage collection determines that there are no more references to the object.
`final Class<?>` | `getClass()` Returns the runtime class of this `Object`.
`int` | `hashCode()` Returns a hash code value for the object.
`final void` | `notify()` Wakes up a single thread that is waiting on this object's monitor.
`final void` | `notifyAll()` Wakes up all threads that are waiting on this object's monitor.
`String` | `toString()` Returns a string representation of the object.
`final void` | `wait(long timeout, int nanos)` Causes the current thread to wait until another thread invokes the `notify()` method or the `notifyAll()` method for this object, or some other thread interrupts the current thread, or a certain amount of real time has elapsed.
`final void` | `wait(long timeout)` Causes the current thread to wait until either another thread invokes the `notify()` method or the `notifyAll()` method for this object, or a specified amount of time has elapsed.
`final void` | `wait()` Causes the current thread to wait until another thread invokes the `notify()` method or the `notifyAll()` method for this object.

## Public constructors

### MutableCallSite

    public MutableCallSite (MethodType type)

Creates a blank call site object with the given method type. The initial target is set to a method handle of the given type which will throw an `IllegalStateException` if called.

The type of the call site is permanently set to the given type.

Before this `CallSite` object is returned from a bootstrap method, or invoked in some other manner, it is usually provided with a more useful target method, via a call to `CallSite#setTarget(MethodHandle)`.

Parameters
`type` | `MethodType`: the method type that this call site will have

Throws
`NullPointerException` | if the proposed type is null

### MutableCallSite

    public MutableCallSite (MethodHandle target)

Creates a call site object with an initial target method handle. The type of the call site is permanently set to the initial target's type.

Parameters
`target` | `MethodHandle`: the method handle that will be the initial target of the call site

Throws
`NullPointerException` | if the proposed target is null

## Public methods

### dynamicInvoker

    public final MethodHandle dynamicInvoker ()

Produces a method handle equivalent to an invokedynamic instruction which has been linked to this call site.

This method is equivalent to the following code:

>     MethodHandle getTarget, invoker, result;
>      getTarget = MethodHandles.publicLookup().bind(this, "getTarget", MethodType.methodType(MethodHandle.class));
>      invoker = MethodHandles.exactInvoker(this.type());
>      result = MethodHandles.foldArguments(invoker, getTarget)

Returns
`MethodHandle` | a method handle which always invokes this call site's current target

### getTarget

    public final MethodHandle getTarget ()

Returns the target method of the call site, which behaves like a normal field of the `MutableCallSite`.

The interactions of `getTarget` with memory are the same as of a read from an ordinary variable, such as an array element or a non-volatile, non-final field.

In particular, the current thread may choose to reuse the result of a previous read of the target from memory, and may fail to see a recent update to the target by another thread.

Returns
`MethodHandle` | the linkage state of this call site, a method handle which can change over time

See also:

- `setTarget(MethodHandle)`

### setTarget

    public void setTarget (MethodHandle newTarget)

Updates the target method of this call site, as a normal variable. The type of the new target must agree with the type of the old target.

The interactions with memory are the same as of a write to an ordinary variable, such as an array element or a non-volatile, non-final field.

In particular, unrelated threads may fail to see the updated target until they perform a read from memory. Stronger guarantees can be created by putting appropriate operations into the bootstrap method and/or the target methods used at any given call site.

Parameters
`newTarget` | `MethodHandle`: the new target

Throws
`NullPointerException` | if the proposed new target is null
`WrongMethodTypeException` | if the proposed new target has a method type that differs from the previous target

See also:

- `getTarget()`

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